1. Field of the Invention
The present invention relates to a focus detecting photoelectric device and, more particularly, to an improvement in the arrangement of a focus detecting photoelectric device. It also relates to a focus detecting system, employing the improved focus detecting photoelectric device, for use in an automatic focus control system of a photographic or television camera, or the like.
2. Description of the Prior Art
In general, the spatial frequency of the light which makes up an image projected through an objective lens assembly is known to have such properties that, when the image is sharply focused on an image plane of the objective lens assembly an amplitude of an a.c. component on the image plane attains a maximum value and that the amplitude of the spatial frequency varies considerably when the spatial frequency in question is within a relatively high frequency range. Various spatial filters, or focus detecting photoelectric devices of contrast detecting type, utilizing the above described properties have heretofore been developed.
In order to detect the high frequency component of brightness of the image, it is necessary to dispose a plurality of light sensors regularly on a plain side-by-side to each other to define an array of light sensors, and at the same time, the distance between the neighboring two sensors should be as small as possible to obtain the a.c. signal with a high frequency. From this point of view, it is concluded that the focus detecting photoelectric device can be formed on a plain substrate.
According to the conventional focus detecting photoelectric devices an in-focus signal is obtained by taking a difference in outputs received from two sensors located next to each other, or every other sensor positions. To meet this end, the sensors, which are formed by silicon photodiodes on a single integrated circuit, are connected in series with each other, and a junction between two sensors is connected to an external circuit which receives a current representing a difference between the photocurrents generated in the two sensors. The detail of this arrangement and its drawback are described below with reference to FIGS. 1 and 2.
In FIG. 1, two silicon photodiodes shown are formed on an n-type single crystal silicon base N0. The first silicon photodiode PDa is defined at a junction between n-type layer N1 and p-type layer P1, and the second silicon photodiode PDb is defined at a junction between n-type layer N2 and p-type layer P2. An electrode is mounted on each of layers N1, P1, N2 and P2, so as to electrically connect layer N1 with a source of positive voltage through a terminal T1, layer P1 with layer N2, and layer P2 with a source of negative voltage through a terminal T3. A junction between the layers P1 and N2 are further connected to a terminal T2 for the external connection. The above connection presents a series connection of two photodiodes PDa and PDb between the positive and negative voltage sources.
In addition to the above, in order to electrically separate the photodiodes on the silicon base N0, an insulation layer N3 is formed between the photodiodes PDa and PDb. The employment of such an insulation layer N3, however, results in the formation of unwanted photodiodes PDx and PDy parasitically between the layer P1 and the base N0 and between the layer P2 and the base N0. Therefore, when light beams impinge as shown by arrows on the focus detecting photoelectric device described above, photocurrents are generated at the photodiodes PDa and PDb, and also at the photodiodes PDx and PDy, as explained below.
Referring to FIG. 2, there is shown an equivalent circuit of the focus detecting photoelectric device of FIG. 1. As understood from the drawing, the photodiodes PDa and PDb are connected in series and are reversely biased by a power source (not shown).
When the image is sharply focused on the detecting photoelectric device, the brightness of the image varies greatly between various points on the image. Assuming that the photodiode PDa is receiving lights which are brighter than those on the photodiodes PDb, the photodiodes PDa and PDb generate photocurrents ip1 and ip2 which are in relation to the brightness of the impinged light, respectively. Since the lights impinging on the photodiode PDa is brighter, the generated photocurrent ip1 is greater than the photocurrent ip2. Therefore, a current difference ip1-ip2 therebetween flows outwardly from the junction between the photodiodes PDa and PDb towards an external circuit connected to the terminal T2. In this case, since the parasitic photodiodes PDx and PDy are also receiving the lights, and since the photodiodes PDx and PDy are, respectively, connected forward and reversely with respect to the difference current ip1-ip2, some percentage of the difference current ip1-ip2 leak out through the parasitic photodiodes PDx and PDy towards the terminal T3 when the parasitic photodiode PDy is receiving light. Thus, a true difference current ip1-ip2 can not be applied to the external circuit.
On the other hand, if the photodiode PDb is receiving lights which are brighter than those on the photodiodes PDa, the generated photocurrents are such that the photocurrent ip1 is smaller than the photocurrent ip2. Thus, a difference current ip2-ip1 flows into the junction between the photodiodes PDa and PDb from the external circuit through the terminal T2. In this case, an additional current caused by the parasitic photodiode PDy flows through the terminal T2 from the external circuit, when the parasitic photodiode PDy is receiving light brighter than that on the parasitic photodiode PDx. Thus, a correct difference current ip2-ip1 can not be derived from the external circuit.
Furthermore, when the image formed on the focus detecting photoelectric device is out of focus, the image is vague, and thus the brightness of the image is approximately the same between two neighboring portions where the photodiodes PDa and PDb are located. Assuming that the light beams having the same brightness are impinging on the photodiodes PDa and PDb, no difference current is generated and, thus no current will flow through the terminal T2 from or towards the external circuit. However, in this case, by the lights impinging on the photodiode PDy, current path is established through the photodiodes PDx and PDy towards the terminal T3 for effecting the current flow from the external circuit through the terminal T2, photodiodes PDx and PDy to the terminal T3.
According to the conventional focus detecting photoelectric device as described above, parasitically formed photodiodes PDx and PDy produce, when they receive lights, unwanted photocurrents which adversely affect difference current generated between the photodiodes PDa and PDb. Thus, the conventional focus detecting photoelectric device fails to provide an exact difference current to the external circuit, which detects the degree of focus by the use of the difference current. Therefore, for the focus detecting photoelectric device formed by the single crystal silicon substrate N0, there is a limit in the accuracy of detection.
Furthermore, according to the focus detecting photoelectric device described above, it is necessary to provide an insulation layer N3 between the photodiodes PDa and PDb to effect the serial connection of the photodiodes PDa and PDb. When such an insulation layer N3 is narrow in the distance between the photodiodes PDa and PDb, the parasitic photodiodes PDx and PDy generate the photocurrent greatly and, thus the insulation layer N3 should have a width greater than a certain width. Such a wide insulation layer N3, however, results in a wide space between the neighboring two photodiodes PDa and PDb. Thus, the spatial frequency of a sampled a.c. signal necessary for the focus detection is limited to a certain level.
The above described focus detecting photoelectric device is disclosed in detail in U.S. Pat. No. 4,039,824 to Nanba issued Aug. 2, 1977, and a similar focus detecting photoelectric device is disclosed in Japanese Patent laid open to public (Tokkaisho) No. 52-70829.